Sunday, January 11, 2015

Cisco CCNP - 300-101 - OSPF Advance Topics

So it's that time again for me to renew my Cisco Certifications. As a result, this post is based on my preparation for the CCNP Route Exam (300-101).

In this post I will be focusing on Advance OSPF Topics

    - Any network change generate LSA
    - LSA sent via multicast
    - Route decision made on total cost of path
    - Router has separate LS Database for each area
    - All routers sharing the same area have the same database
    - router id must be unique within the OSPF domain
    - Areas are 32-bit number
    - - reserved for Backbone
    - All areas must connect through area 0
    - Uses TOS value of 0
    - Does not support route summarization on routers other than ABR or ASBR
    - Supports equal cost load balancing but not unequal cost load balancing

    - Uses Link State Protocols
    - Shortest Path First Algorithm (SPF)
    - Link State Database
        - Created from Link State Packets
    - Path Database   
        - ID, Path cost, direction
    - Forwarding database
        - Routing table
    - All routers exchange Link State Packets (LSP)
    - Each router first consider itself as root
OSPF Packet Types
    - Hello
        - Multicast to on all router interfaces
        - Multicast to used for DR and BDR
        - Discover neighbors
        - Sent every 10 seconds on Broadcast 30 seconds on NBMA
        - Used for Adjacencies

    - Database Description
        - Link state database
        - Describe the current LSDB
    - Request
        - Request part of the database
        - Response should contain only the requested update
    - Update
        - Contains detailed LSAs
        - sent in response to LSR
    - Acknowledgment
        - confirm receipt  of an LSU
LSA Types (20 byte header)
        - Type 1 - Router Link   
            - Created by each router to represent itself in each area
            - State and cost of router's links in the area
            - All links must be in a single LSA
            - Only flooded in the specific area
            - Router states whether its is ASBR, ABR or end point of  virtual link
        - Type 2 - Network Link
            - found on broadcast networks and non broadcast network
            - only sent by designated router originates LSA
            - describes all routers attached to the network
            - flooded only through the area and more

        - Type 3 - Network Summary
            - destinations outside of the area but still within the AS
            - Originated by ABR
            - Only Intra-area routes are advertised   
            - type 4 is the ASBR information
            - routes displayed as "O IA"
            - LSA type 4 with mask
        - Type 4 - ASBR
            - destinations outside of the area but still within the AS
            - Originated by ABR
            - Only Intra-area routes are advertised
            - Information about ASBR
        - Type 5 - External
            - Routes external to the AS
            - Default route is sent as external
            Types of external LSA
                - E1 - Total cost up to external destination
                - E2 - Considers only the cost of the outgoing interface to external destination
        - Type 7 - NSSA External
Designated Routers
    - Reduces traffic on Broadcast networks
    - Store and distribute neighbor information
    - Has a backup DR for redundancy
    - Uses Priority in the range 1-255, 0 is for non candidate
Types of Areas:
    - Normal
        - External links injected
    - Stub
        - Summary LSA from other areas injected
        - LSA type 5 not injected
        - Define all routers in an area as stub
        - Reduce overhead and LSA counts
    - Totally stubby (stub no summary)
        - Represents all external links
        - non-summarized internal links
    - Not so stubby (NSSA)
        - Benefits of stub area
        - when flooded into area 0 LSA type 5 become LSA type 7
    - Done by the longest prefix
    - One summary prefix is advertised

Neighbor states
    - Down
        - No information received from neighbor
    - Attempt
        - No information received from neighbor but effort made to contact neighbor
        - Used when "neighbor" command is used
        - if stuck in attempt, neighbor hellos may be lost
    - Init
        - Hello packet received but router itself not listed in the packet
        - if stuck in Init state, this means one way hello messages
    - 2-Way
        - Two way communication established. Beginning of adjacency
        - stuck in 2-way is normal on broadcast networks
    - Extart
        - Determines which router will be master/slave
        - Determines the first sequence number
        - if stuck in extart check neighbor RID
    - Exchange
        - Link state is describe through the DBD packet
        - Each sequence is ack'd
        - Only one DBD packet is allowed
        - Linkstate packets are sent to request newer LSA
        - if stuck in exchange check neighbor RID
    - Loading
        - Packets are requested for the most recent LSA which were not received during the exchange
        - if stuck in loading LS request being sent but neighbor ignoring. Also check MTU mismatch
    - Full
        - All information has been exchanged
Manipulating OSPF
    - Change the reference bandwidth
        - uses "auto cost referemce bandwidth"
        - Cisco recommends setting the value to the same on all routers
    - setting the interface bandwidth
        - uses "bandwidth" command
    - setting the OSPF cost
        - uses "ip ospf cost"
Commands to verify OSPF
    - show ip ospf interface brief
    - show ip ospf neighbor
    - show ip ospf database
    - show ip ospf database network
    - show ip ospf database summary
    - show ip route ospf


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